We have been studying the biochemistry and genetics of lambda site-specific recombination. We have purified the proteins involved in excisive site-specific recombination of bacteriophage lambda. We are studying the role of the phage-coded functions, Xis and Int, and host-coded functions in this recombination reaction. We are extending these studies to the in vitro recombination reaction. We are isolating mutations in Int and Xis which alter the interaction of these proteins during integration and excisive recombination.